A mammal must judiciously partition its available energy among many competing reproductive and non-reproductive needs. We know little about the nature of this partitioning process, either metabolically or neuroendocrinologically. It is the intent of this proposal to learn much more about this process by exploiting a unique kind of naturally-occurring variation that occurs in wild rodents. Some temperate zone populations of rodents contain a mixture of genetically-different individuals that use markedly different reproductive strategies; some individuals rely on photoperiod to time their annual reproductive cycle while others in the same population ignore this cue, breeding opportunistically instead, sometimes even during the winter. This is a proposal to elucidate the genetic, behavioral, metabolic and neuroendocrine bases of these alternative strategies using voles of the genus Microtus as a model. Of particular concern will be the kinds of energetic adaptations that allow opportunists both to survive and reproduce during the winter. First, wild-trapped voles will be subjected to selection to develop two distinct genetic lines of animals, one reproductively-photoresponsive and one non-responsive/opportunistic. Second, the reactions of these two genetic lines to the two energetic challenges of winter, food shortage and low temperature, will be compared. Daylength will also be a variable in these experiments. Of core concern here are potential differences in the voles ability to survive and maintain their capacity to reproduce. Incorporated therein, however, are interests in a variety of specific energy-related behavioral and metabolic adaptations, as well as whole body energy balance. Third, the reproductively important neuroendocrine reactions of the two genetic lines to food shortage and low temperature will be compared. One result of this research will be a solid set of principles that pertain to the metabolic process wherein assimilated energy is allocated to competing reproductive and non-reproductive needs. A second result will be an unparalleled set of genetic tools with which to develop still greater understanding of this important process.